CHAPTER II 

 THALLOPHYTA: ALGAE 



The thallophytes comprise a large and diverse assemblage of simple 

 plants forming the lowest division of the plant kingdom. They number 

 about 88,000 species. The plant body may be unicellular but, where 

 multicellular, as is generally the case, it is a thallus — a body without 

 differentiation into true vegetative organs, such as characterize the higher 

 plants. This distinction is not absolute, however, as some of the marine 

 algae have parts that superficially resemble true vegetative organs, while 

 some of the bryophytes have thallus bodies. A more tenable distinction 

 is based on the structure of the reproductive organs. The sporangia of 

 thallophytes, with only a few exceptions, are unicellular; those of the 

 higher plants are always multicellular. The gametangia of thallophytes 

 are prevailingly unicellular but, where multicellular, have no outer layer 

 of sterile cells (except in the Charophyceae). In the thallophytes the 

 zygote does not produce an embryo within the female sex organ, as it 

 does in all the higher groups. 



The Thallophyta include two main series, the algae (Phycophyta) and 

 the fungi (Mycophyta), the former with 18,000 species and the Latter with 

 70,000. The algae, having chlorophyll, are able to make food by photo- 

 synthesis and so are independent {autotrophic) plants. The fungi, lack- 

 ing chlorophyll, must obtain their food from an external source and so 

 are dependent (heterotrophic) plants. This distinction, being physiologi- 

 cal, is a convenient one but does not necessarily express relationship; 

 thus it may be without phylogenetic significance. For this reason the 

 various classes of algae and fungi are often regarded as separate and more 

 or less coordinate groups of thallophytes rather than as members of two 

 different series. 



Algae live in both fresh and salt water, while a few grow on moist soil, 

 wet rocks, tree trunks, or in other terrestrial habitats. They include the 

 pond scums, kelps and other seaweeds, and a host of less famihar forms. 

 Many are microscopic, but some kelps reach a large size. Because of 

 their perishable nature, algae have left few reliable records of their exist- 

 ence during geologic times. Most of those preserved as fossils are lime- 

 secreting seaweeds and forms with sihceous shells (diatoms). As here 

 presented, the algae are distributed among 10 main classes, the Cyano- 

 phyceae, Euglenophyceae, Chrysophyceae, Dinophyceae, Xanthophy- 



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